Prediction Method And Application Of Overburden Fracture Development Range Of Underground Coal Gasification Without Well

Underground coal gasification is an important technical means for the coal industry to achieve the goal of carbon peak carbon neutralization.At present,China ’s shaftless UCG process and combustion control technology have reached the international leading level,laying a foundation for the large-scale production of underground coal gasification.However,with the popularization and application of industrialization,how to solve the fracture of surrounding rock of underground gasifier and its induced groundwater intrusion will become the core technical problem hindering its normal production,which greatly limits the industrialization of underground coal gasification.The key to solve the above problems is to clarify the development form of overburden fracture in underground coal gasification and calculate the range of fracture development,so as to take measures to prevent it in advance.Therefore,this thesis takes the Wulanchabu underground coal gasification field as the research object,and uses theoretical analysis and numerical simulation methods to analyze the development morphology and evolution characteristics of overburden fractures in underground coal gasification without wells.The main research results are as follows :(1)The study reveals the evolution characteristics of the fracture development of the overburden rock in the UCG without well from ’ rectangle ’ to ’ butterfly airfoil ’.With the advancement of the non-well underground coal gasification working face,the development form of the overburden fracture in the combustion space gradually changed from ’rectangle ’to ’saddle-shaped ’,and finally developed into ’butterflyshaped ’,which was significantly different from the ’saddle-shaped ’development of overburden fracture in conventional underground mining.At the same time,when the gasification working face advances for a certain length,the development form and height of the overburden rock fracture in the combustion space area will not change,that is,there is a limit value for the development height of the overburden rock fracture in the combustion space area,which provides a theoretical basis for predicting its development height.In addition,the stress of overlying strata with different combustion cavity shapes in underground coal gasification without shaft is analyzed,and the initial caving step distance of overlying strata is deduced,which lays a foundation for the design of gasification working face and the analysis of the influence of gasifier width on the fracture development of overlying strata.(2)The size of the working face,the width of the isolated coal pillar and the mining height have a significant impact on the height of the overlying rock fracture development in the underground coal gasification goaf.There is a power function relationship between the height of overburden fracture development in the combustion area and the advancing length of the gasification working face,and when the working face advances to a certain extent,the height of overburden fracture development will not increase with the advancement of the working face.The development height of the overburden rock fracture in the combustion zone is linearly positively correlated with the width of the gasifier and the mining height.At the same time,when the middle isolation coal pillar is stable,the increase of the width of the isolation coal pillar will not affect the development height of the overburden rock fracture in the combustion zone.Finally,the normalized method is used to unify the factor dimension units of each fracture development height,and it is concluded that the width of underground gasifier has the greatest influence on the height of overburden fracture development,which lays a foundation for the construction of prediction method of overburden fracture development height in underground coal gasification.(3)The development law of overburden fracture morphology under the same recovery rate and different recovery rate is analyzed.The results show that when the isolated coal pillar is stable and the width of the combustion zone is small,the fracture development on both sides of the combustion zone of underground coal gasification is not obvious,and the fracture development shape is ’ rectangular ’.When the recovery rate is greater than or equal to 50 % and the same width of the isolated coal pillar,the height of the fracture development of the overburden increases with the increase of the recovery rate,which lays a foundation for the design of the isolated coal pillar and the working face.(4)The prediction model of overburden rock fracture development height in underground coal gasification combustion area is established by least square method.At the same time,according to the relationship between the width of the gasifier and the height of the fracture development of the overlying rock,based on the calculation method of the development height of the water-conducting fracture zone in conventional underground mining,the calculation method of the height of the fracture development of the overlying rock in the gasification combustion zone is constructed for the practical application of the project.Under the premise of ensuring the safety of gasification,the remaining mining areas in Wulanchabu mining area were designed.